The art of treating edible oil with alkali has heretofore been carried out in slurry tanks using gentle agitation with residence times of from 3 to 40 minutes. These extended residence or contact times were believed to be necessary to accomplish the desired dewaxing of refined oil or refining of crude oil. E.g., U.S. Pat. No. 3,943,155 to Young, Mar. 9, 1976, discloses a process for refining and dewaxing crude vegetable oils using only one separation step which removes both the hydrophilic and waxy components from the crude oil. Aqueous alkali is agitated with the crude oil in a "crystallizer tank" at a temperature of from about 15.degree. F. to about 45.degree. F. for 15-40 minutes to form an emulsion. The emulsion is broken by mixing it with phosphoric acid. U.S. Pat. No. 4,035,402 to Levine, July 12, 1977, discloses a process for effectively dewaxing vegetable oils which comprises: chilling a refined and water-washed or filtered vegetable oil, mixing the chilled vegetable oil with a dilute alkaline solution, gently agitating the resulting mixture in a low shear, high circular manner for at least 30 minutes, centrifuging the mixture and separating a heavy phase which contains wax and other impurities from a light oil phase.
In general, crude vegetable glyceride oils, as they are obtained from their natural sources by conventional extraction methods, normally contain various nonglyceride impurities. These nonglyceride substances include gross material from the source of fat, such as xanthophyll or chlorophyll; products obtained by the breakdown of the glyceride during treatment, such as free fatty acids; and other derivatives of the glycerides, such as phosphatides and sterols. In addition, many vegetable oils normally contain natural waxes. Some of these above-mentioned impurities are desirable in that they help to protect the oil from oxidation or other adverse processes, but by far the greater amount of these substances must be removed during processing for edible purposes because they are deleterious to the appearance, taste, keeping qualities, and other properties of the oil. Refining and winterizing or dewaxing operations have, thus, become commonly employed to effect the removal of these impurities. As used herein treating edible oil with alkali includes reacting the alkali with these impurities to remove them.
The removal of gross impurities, gummy or mucilaginous material, and the free fatty acids from the glyceride oil is commonly referred to as "refining" and as used herein the term includes refining and bleaching or refining as these terms are used in the trade. In a typical refining operation, undesirable impurities are preferentially combined with a refining agent to form hydrophilic components. These are subsequently removed from the oil by a separation of aqueous and oil phases. Known methods of refining include contacting the glyceride oil with strong or dilute alkaline materials followed by separation of impurities, by liquid-liquid extraction of impurities from the glyceride oils, by steam distillation, and by contacting the glyceride oils with acids. Each of these methods is said to have its advantages for use in refining oil of one type or another for a certain ultimate utility by removing to a greater or lesser extent the hydrophilic component of the oils.
However, these known refining methods do not remove all of the impurities from the vegetable oil, and in particular, waxy components tend to remain in such oils. For example, if the refined vegetable oils are cooled to a temperature of about 4.degree. C., the higher melting triglycerides and any vegetable waxes (linear esters) present will crystallize and either impart a cloudy appearance to the oil or settle out as a crystalline precipitate. When the oil is again raised to room temperature, the crystallized waxes may or may not redissolve in the oil. Thus, the oil at room temperature may or may not regain its clarity depending upon the amount of the respective impurities contained in it. Thus, without further processing, any such vegetable oil containing these higher melting triglycerides or vegetable waxes is not suitable for certain purposes where the clarity of the oil at low temperatures is important.
For example, oils which are suitable for salad oil use frequently are stored in refrigerators. The prolonged cooling of such oils to temperatures normally encountered in refrigerators, such as from about -1.degree. C. to about 10.degree. C., requires a product which retains its clarity if it is to be desirable to the consumer.
The ultimate objective of a refining, dewaxing or a combined operation is to remove every undesirable impurity completely, while at the same time maintaining intact all of the desirable glyceride oil. The particular process used with a given oil is determined by the foregoing considerations of maximum impurity removal with the minimum of glyceride oil loss. Since a good part of the refining cost arises from losses of glyceride oil, much work has been done to increase the efficiency of refining and dewaxing operations, and many processes have been developed for this purpose. The majority of the refining processes developed employ temperatures of at least room temperature and often higher to obtain a complete removal of the hydrophilic impurities and to minimize oil losses. Refining processes, of course, do not accomplish removal of all of the free fatty acids or all of the waxy components, which are inseparable from the glyceride component at these higher temperatures.
Thus, a separate low-temperature dewaxing step is necessary to remove the waxy components. Since there are oil losses inherent in the separation steps which usually follow the refining and dewaxing operations, methods which embody multiple separation steps tend to be uneconomical. Low-temperature refining methods have been attempted to simultaneously remove the hydrophilic and waxy components from the crude oil. However, the methods developed thus far have not been entirely satisfactory. At low temperatures, a virtually inseparable emulsion tends to be formed from a vegetable oil and an aqueous refining agent. This results either in extraordinarily high oil refining losses or an incomplete removal of the impurities, the latter of which results in a cloudy oil at low temperatures.
Today, much of the glyceride oil is refined in a continuous process. This process involves the steps of bringing the oil and alkali to an elevated temperature, mixing these two materials, adjusting the temperature, if necessary, providing a sufficient hold time, and continuously separating the refined glyceride oil from the impurities by centrifugation. Thereafter, if dewaxing is necessary, the refined oil is cooled to a low temperature to crystallize the waxy components. These are then removed by either a slow filtration or a second aqueous separation step performed on the cooled oil.
U.S. Pat. No. 3,704,006 to Grout et al., granted Nov. 28, 1972 relates to a method for producing a dispersion of a dispersed phase in a continuous phase field in which the dispersed phase is immiscible. The method is performed in a system which includes a conduit containing a plurality of sheet-like elements extending longitudinally within the conduit. Each element is curved to turn the direction of fluid flowing past it. The elements are arranged in alternating right- and left-handed curvature groups (a group consisting of one or more elements). The two phases are injected into the conduit and pumped through it at predetermined velocity, which together with the density of the continuous phase, the interfacial tension between the phases and the inner diameter of the conduit determines the Weber number. It is taught that the drop production reaches an equilibrium between break up and coalescence at about twelve elements and is well stabilized at 21 elements. It is taught that the system and method can be used to extract solvents, remove color from or clarify liquids, remove or add heat, or affect mass transfer rates in reactions. Acidic or alkaline solutions to be combined with organic liquids are mentioned. It may completely oxidize a contaminant in an effluent, or for example, by dispersing an oxygen containing gas in water in which is dissolved NA.sub.2 SO.sub.3. Tests on various hydrocarbons dispersed in water are reported.
Grout et al. does not, however, teach treating edible oils with aqueous alkali solutions in processes comprising dewaxing refined oils, refining crude oils or combined refining and dewaxing crude oils. It remained for the present inventor to bring those processes to light within the context of treating edible oil with aqueous alkali using an interactive surface mixer.
In view of the above, it is an object of this invention to provide an improved method of treating edible oil with an aqueous alkali solution to provide an easily separable mixture of a reacted aqueous alkali phase and a reacted oil phase.
Another object of the present invention is to reduce neutral oil loss in processing edible oil with an aqueous alkali solution.